Integrated electrical circuits and other microstructured components are conventionally manufactured by several structured layers being applied onto a suitable substrate, which may be, for example, a silicon wafer. For the purpose of structuring the layers, the latter are firstly covered with a photoresist that is sensitive to light pertaining to a certain wavelength region—for example, light in the deep ultraviolet spectral region (DUV). Subsequently the wafer that has been coated in this way is exposed in a microlithographic projection exposure apparatus. In this process a pattern of structures which is arranged on a mask is imaged onto the photoresist with the aid of a projection objective. Since the reproduction scale in this process is generally less than 1, projection objectives of such a type are frequently also designated as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching process, as a result of which the layer is structured in accordance with the pattern on the mask. The photoresist left behind is then removed from the remaining parts of the layer. This process is repeated until all the layers have been applied onto the wafer.
The performance of the projection exposure apparatus that are used is determined not only by the imaging properties of the projection objective but also by an illumination system that illuminates the mask with the projection light. For this purpose the illumination system contains a light source, for example a laser operated in pulsed mode, as well as several optical elements which generate, from the light generated by the light source, light bundles that converge on the mask at field points. In this connection the light bundle desirably has certain properties, which in general are matched to the projection objective.
These properties include, inter alia, the angular distribution of the light bundles which each converge to a point in the mask plane. The term ‘angular distribution’ describes how the total intensity of a light bundle is distributed to the different directions from which the individual rays of the light bundle impinge on the relevant point in the mask plane. If the angular distribution is specially adapted to the pattern contained in the mask, the pattern can be imaged onto the wafer covered with photoresist with higher imaging quality.
In illumination systems for microlithographic projection exposure apparatus the use of multi-mirror arrays (MMAs, also referred to as micromirror arrays or mirror matrices), which include a multitude of individually drivable micromirrors in order to deflect individual partial beams of the projection light of the illumination system in different directions, has recently been taken into consideration. For example, in this way the respective partial light-beams of the projection light can be directed to different locations in a pupil plane of the illumination system with the aid of the micromirrors. Since the intensity distribution in a pupil plane of the illumination system crucially influences the angular distribution of the projection light, the angular distribution can be set more flexibly by individually tilting the micromirrors. Particularly in connection with so-called unconventional illumination settings, in which an annular region or several poles in the pupil plane are illuminated, the use of MMAs can enable the angular distribution to be adapted to the respective circumstances—in particular, to the mask to be projected—without, for example, diffractive optical elements having to be exchanged.
Such MMAs are frequently produced as microelectro-mechanical systems (MEMS) by lithographic processes such as are known from semiconductor technology. The typical structure sizes with this technology amount in some cases to a few micrometers. Known representatives of such systems are, for example, MMAs having micromirrors that can be digitally tilted about an axis between two end positions. Such digital MMAs are used in digital projectors for the reproduction of images or films.
However, for the application in the illumination system of a microlithographic projection exposure apparatus it is desirable for the micromirrors to be capable of assuming—quasi-continuously and with a high precision—any tilt angle within an angular working range. The actuators that bring about the tilting of the micromirrors may in this case have been constructed, for example, as electrostatic or electromagnetic actuators. Thus in the case of known electrostatic actuators the tilting of the micromirror is based, for example, on the fact that a fixed control electrode and a mirror electrode fitted on the rear of a micromirror attract variably strongly, depending upon the applied voltage. Via a suitable suspension and several actuators the micromirror can consequently be tilted by arbitrary tilt angles.
By reason of high demands made of the precision in the course of tilting the micromirrors, it is desirable for the actuators to be driven extremely precisely by drive electronics. In this connection it is to be observed that by virtue of the plurality of individual mirrors, for example 1000, in an MMA—which are usually driven with the help of several actuators per mirror—it is desirable for such drive electronics to be designed efficiently.
In particular, for example for the purpose of driving an MMA with electrostatic actuators of drivers arranged in an extremely small space, it is desirable for a multitude of different output-voltage values to be generated with high voltage. According to aspects in the state of the art, the desired output voltage is generated in a conventional driver circuit, a so-called Class-A circuit, with the aid of transistors and resistors which are fed with a high voltage by way of supply voltage. Owing to the principle, however, in a Class-A circuit a static leakage current invariably flows to earth through the transistor and the resistor. As a result a considerable dissipation of power can arise in conjunction with evolution of heat. For reasons of power dissipation, it is therefore desirable for the resistors to be highly resistive, with the result that they may occupy a large space within the driver and hence within the drive electronics. In the case of the drive of larger MMAs with over 1000 mirrors, this has a particularly disadvantageous effect, since the drivers are desirably positioned as close as possible to the MMA in an extremely small space.
One possibility to encounter the problems that have been outlined above is described in U.S. Pat. No. 6,940,629 B1. The drive of an MMA mirror is undertaken therein via an integrating driver stage in which an output current is integrated on an external and/or internal capacitor. The output current in this case is proportional to a reference current which is generated from a digital value by digital-to-analogue conversion. In this connection the digital value is generated by a monitoring unit which is not specified in any detail in the patent. According to this publication the setting of the output voltage for an actuator of the micromirror is obtained by a reference current of adjustable duration being supplied to an integrator via a high-voltage element and by the desired voltage for adjusting the respective micromirror being established by the choice of the duration of the flow of current. In this case the duration of the flow of the reference current is established via digital control signals.